1. Field of the Invention
The present invention relates generally to the inspection of electrical generators and, more particularly, to a mount for installing a search coil in or about the air gap between the rotor and the stator of an electrical generator without requiring removal of the rotor.
2. Background Information
Electrical generators, such as, for example, large turbine generators, must be routinely inspected to assure optimum performance and safe operating conditions.
FIGS. 1 and 2 show a representative example of a turbine generator 2. The generator 2 generally includes an outer housing 4 which encloses an annular stator 6 and a rotor 8. Typically, the housing 4 is airtight and contains a coolant gas, such as hydrogen, which is circulated throughout the generator 2. The annular stator 6 is supported in a frame 10 within housing 4 and includes a stator or armature winding 12 which is disposed in slots 14 (FIG. 2) in the stator 6. The rotor 8 is generally positioned concentrically within the stator 6 and includes rotor winding 16 which comprises a multi-turn winding 16 that is distributed in slots 18 in the cylindrical rotor 8, as shown in FIG. 2. In this manner, a two-pole or four-pole magnetic field is created in the air gap 20 between the stator 6 and the rotor 8. The rotor winding 16 must be inspected for faults, such as electrical shorts between turns of the winding 16.
In such a winding, each pole of the winding is essentially the same as the other pole or poles. Therefore, the waveform of the flux wave produced in the air gap by each pole, is the same. However, if for example, two or more turns in one pole of the winding become shorted together, or if a ground fault occurs in the winding, this symmetry is destroyed and the flux waves of the different poles are no longer identical. As a result of the asymmetry in the winding due to the fault, the field flux waveform will contain even harmonics. Accordingly, the occurrence of even harmonics in the air gap flux wave is a reliable indicator of the existence of a fault in the rotor winding. Monitoring the field flux for even harmonics is typically accomplished using a search coil or coils (see, e.g., search coils 22, 24 of FIG. 2) suitably disposed in the air gap in a position such that the plane of the search coil is substantially perpendicular to the radial magnetic flux in the air gap. Use of search coils as fault detection apparatus for the rotary winding of rotary machines are described in further detail in, for example, U.S. Pat. Nos. 4,136,312, 4,377,784, and 4,667,148.
However, among the disadvantages associated with known search coil installations are the fact that they are complex in design, require multiple components, and generally require that the rotor be removed from the generator in order to install the search coil, that the search coil or flux probe be pre-installed at the time of initial manufacture of the generator, or that a substantial modification be made to the generator subsequent to its manufacture. See, e.g., U.S. Pat. No. 4,667,148. Complex and numerous components are cost intensive as is the need to substantially modify the generator in order to install the search coil, and removal of the rotor disadvantageously requires a prolonged maintenance outage. It is, therefore, desirable that the search coil be readily installable in existing generators already in operation. It is further desirable to provide a search coil mount which allows quick and easy installation of the search coil and inspection of the rotor winding with the rotor in situ.
There is, therefore, room for improvement in search coil mounts for electrical generators.